https://github.com/mozilla-mobile/firefox-ios/pull/16545

Reader mode loads the page from a local copy cached from the non-reader mode connection; via localhost, which doesn't support https. (or rather, uses a self-signed cert which appears insecure). As this connection is entirely internal to the device, proper https isn't necessary.

Later versions of the app will hide that icon in reader mode, but it doesn't look like that's made it to android yet.

There's a ticket in the Firefox bug tracker that appears to be tracking this: https://bugzilla.mozilla.org/show_bug.cgi?id=1913601

Makes sense, because even in reader mode it shows https, and the issue happens on other major sites like Wikipedia.

So what's the problem according to FF?

Check your time and date.

The website itself doesn't seem to have a problem with its certificate: https://www.ssllabs.com/ssltest/analyze.html?d=www.fosslinux.com&s=155.138.208.156&latest=

it might be related to the root certificates available on your device. you might be able to click the barred icon and read details about the error that's encountered

Apparently it's because it's using a self signed certificate, but I'm having trouble finding exact sources that confirm this.

"connection is not secure"

you can "enable in all tabs" "HTTPS-Only mode", if that bothers you. iirc it's not on by default, which permits to fallback on HTTP

?

edit : i see, reader is not online, i believe. It's an in browser rendering of the site.

Did you check the system clock for accuracy?